The invention concerns an antilocking system for a road vehicle with a hydraulic two-circuit brake installation in which the rear wheel brakes are combined in a static brake circuit, connected to an output pressure space of a brake unit, which can be actuated by means of a brake pedal. The brake unit is capable of generating an output pressure proportional to an actuating force. The output pressure can be connected to the wheel brakes of this static rear axle brake circuit by means of an electro-hydraulic brake pressure control valve arrangement provided for controlling brake pressure reduction, brake pressure retention and brake pressure restoration phases. The antilocking system operates on the pump-back principle according to which, in a brake pressure reduction phase, brake fluid drained from a wheel brake subject to the control system is pumped back by means of a return pump into the output pressure space of the brake unit associated with the static brake circuit. This return pump is designed as a piston pump which is driven hydraulically with its drive pressure space being subjected to and relieved from a pressure under control of a solenoid valve control. The pump chamber piston pump is connected to the wheel brake, currently subjected to the control system, via an inlet non-return valve and, via an outlet non-return valve, to the output pressure space of the brake unit associated with the static brake circuit. The antilocking control takes place on the select-low principle according to which, independent of whether a locking tendency occurs on one or both wheels of the static brake circuit, the brake pressure on both wheel brakes is always kept at the same value and the control signals necessary for the correct control switching of the brake pressure control valve arrangement and the solenoid valve arrangement controlling the pump drive are provided by an electronic control unit. The unit generates these signals by a process of comparing and differentiating--with respect to time--electrical output signals from wheel rotational speed sensors whose output signals, in level and/or frequency, are a measure of the wheel peripheral speeds of the vehicle wheels.
An antilocking system of this general type is known from the German patent specification No. DE 3,119,982,C2 if it is assumed that the brake pressure control valves individually associated with the driven rear wheels of the vehicle are actuated in common. This is explained as being a rational measure from a control technical point of view by the publication Bosch, Technische Berichte, Volume 7 (1980), Number 2, Page 87, right-hand column.
An antilocking system of this general type is also known from No. DE-OS 3,347,618.7-21, although this describes an antilocking control system operating on the pump-back principle which utilizes a hydraulically driven return pump for the front axle brake circuit. In this system, the control operates as a single wheel control system on the select-high principle and a control system on the select-low principle is only explicitly revealed for the rear axle brake circuit, designed in that case, as an open brake circuit.
For the static brake circuit, the known antilocking system provides a low pressure reservoir which quantitatively accepts the brake fluid drained from a wheel brake subject to the control system in a pressure reduction phase. This brake fluid is pumped back by means of the hydraulically driven return pump into the output pressure space, associated with the static brake circuit, of the brake unit. This return pump is designed in such a way that between 0.2 and 0.4 cm.sup.3 of brake fluid are pumped back per piston stroke into the output pressure space of the brake unit, which corresponds to between 1/20 and 1/10 of that quantity of brake fluid which is forced into the rear axle brake circuit in a vehicle of higher power class (vehicle weight approximately 1.5 tons and maximum speed approximately 200 km/h) when braking occurs with the maximum possible brake installation design pressure.
A valve, provided as the brake pressure control valve for the rear axle brake circuit, is a single 3/3-way solenoid valve whose basic position is the brake pressure build-up position. This valve can be driven by a control signal of defined control current strength from the basic position into a shut-off position--the brake pressure retention position--and from there into a brake pressure reduction position by a control signal of defined higher, e.g. double, control current strength. In the brake pressure reduction position, the wheel brakes subjected to the control system are connected to the low pressure reservoir from which the brake fluid is pumped back into the output pressure space of the brake unit by means of the hydraulically driven return pump.
In such a design of the brake installation and the antilocking system, pumping back to meet the requirements required to drive the hydraulic return pump with a stroke repetition frequency of at least 10 Hz leads to unpleasant noise in control operation, because of the associated pressure shocks. Such noise is not only disadvantageous because of the associated adverse effect on driving comfort, but, more particularly, because careful drivers who, due to their "defensive driving style", only cause the antilocking system to respond extremely rarely and are therefore not accustomed to this noise and thus can be startled by it and react even worse to the moment of danger with which a braking situation requiring an antilocking control system is associated.
Also disadvantageous in this known antilocking system is the substantial technical complication necessary to achieve it. This complication is due to the low pressure reservoir necessary, in addition to the hydraulically driven return pump.
The same disadvantage applies analogously, even in the antilocking system operating on the pump-back principle, widely used and known from "Bosch Technische Berichte", Volume 7, 1980, Number 2, in particular Page 87, FIG. 39 or FIG. 40. There the electrically driven return pumps each have to be designed for an output of approximately 10 cm.sub.3 /s if, during a brake pressure reduction phase of the antilocking control system they are to be capable of pumping back the maximum quantity of brake fluid which can be accepted within 20 ms by a low pressure reservoir into the main cylinder of the brake installation within a cycle time of 200 ms demanded by the control system. There a hydraulic pump-back drive is used instead of the electrical pump drive as is intrinsically known from No. DE 3,119,982 C2, which is designed for a comparable output, quoted in No. DE 3,119,982 C2 as 4 cm.sup.3 /s as a minimum value (see Column 7, lines 40-45). The dimensioning of the return pumps given in No. DE 3,119,982 C2 require a switching frequency of 20 Hz for the solenoid valve necessary to control the drive of the return pumps, which again introduces substantial noise for the pump drive.
The variants of an antilocking system known from the German Pat. No. De 3,030,621 A1 are also subject to the disadvantage of disturbing noise. In this system, piston pumps driven by means of an eccentric and individually associated with the brake circuits are provided for the control of the brake pressure restoration phases of the antilocking control system. In contrast to an antilocking system operating on the pump-back principle, these piston pumps direct brake fluid from the brake fluid reservoir into the wheel brake cylinders.
Noises associated with a response of the control system and caused by a pulsating pump drive are substantially avoided by the antilocking system known from the German Pat. No. DE 3,322,422 A1. In this system, individually allocated pressure modulators are provided for the wheel brakes subject to the control system. These pressure modulators have a stepped piston which, within a modulator housing, forms the pressure-tight movable boundary between a modulator space with a larger cross-sectional area and an annular space with a smaller cross-sectional area. The smaller diameter piston step of the modulator piston emerges in a pressure-tight and movable manner from the housing and forms the spindle nut of an electrically driven spindle drive whose actuation, in alternative rotational directions, permits the piston to be displaced in one direction or the other in the housing, so that the modulator spaces experience opposite changes in volume. The spindle drive is designed to be irreversible. The annular space of the particular modulator is permanently connected to an outlet pressure space of the brake unit provided for actuating the brakes. The modulator space with the larger cross-section is permanently connected to the wheel brake cylinder of the wheel brake which can be subjected to the control system. The outlet pressure space of the brake unit, and with it, also the annular space of the particular modulator, can be shut off by means of a solenoid valve against the particular wheel brake and the modulator space associated with it. This solenoid valve is driven into its shut-off position for the duration of the antilocking control.
In order to achieve a reduction in braking pressure in the particular wheel brake, the modulator piston is driven so as to increase the modulator space--by driving its drive motor in the correct rotational direction--so that the modulator space can accept brake fluid from the wheel brake and, at the same time, brake fluid is displaced from the annular space into the brake unit. In order to achieve brake pressure restoration phases, the drive motor of the pressure modulator is driven in the opposite rotational direction so that brake fluid is displaced from the modulator space back into the wheel brake and brake fluid can also flow from the brake unit back into the annular space of the modulator.
A disadvantageous feature, from a constructional point of view, of this known antilocking system is the high level of technical complications resulting from the fact that each modulator must be provided with its own electrical drive motor. From a functional point of view, it can be considered a disadvantage that the feedback effect of the control system, felt at the brake pedal, has a shock-like nature, particularly when the control system is designed for a very rapid response behavior, and is associated with a relatively large amplitude of motion of the brake pedal even if the control system responds relatively gently.
These disadvantages are reduced in the case of an antilocking system described in the German Pat. No. DE 36 37 781 A1, which also operates on the principle of brake pressure change in the wheel brakes subject to the control system by means of volume changes in modulator spaces. To the extent that the pressure modulators used are hydraulically driven, i.e. by a valve-controlled connection to the non-pressurized tank or the pressure output of an auxiliary pressure source provided in common for all the pressure modulators and including a pressure reservoir charging pump, the technical complication is reduced overall. To the extent that the pressure modulators, which are connected between the brake unit and the brake pressure control valves upstream of the wheel brakes, produce no effect on the brake unit as long as the brake pressure reductions necessary in control operation are only moderate, they are operated like return pumps in the case of larger magnitudes of the brake pressure reduction necessary, wherein return pumps then displace brake fluid drained from a wheel brake subject to the control system partially into the outlet pressure space of the brake unit of the brake installation, associated with this wheel brake. By this means, a feedback on the response of the antilocking system, which can be felt on the brake pedal and is uncomfortable to the driver, is limited to those cases in which drastic brake pressure changes are necessary, which is usually the case with particularly bad road conditions about which the driver should also be "informed". In those cases of antilocking system response in which only moderate brake pressure changes are necessary, i.e. in the large majority of braking situations which, although requiring the control system are still "not dangerous", a pedal reaction irritating to the driver is, so to speak, suppressed.
A disadvantageous feature of this antilocking system, however, is the fact that in order to achieve a bypass flow path by means of which braking is still possible even if the auxiliary pressure source has failed, a relatively complicated design of the pressure modulators is necessary and this again leads to increased technical complications.
The object of the invention is therefore to improve an antilocking system of the general type mentioned at the beginning in such a way that noise associated with the response of the antilocking system is substantially avoided but that, nevertheless, there is a clear reduction in the technical complexity necessary to achieve this antilocking system.
The invention achieves this object by having the brake pressure control valve designed as a 2/2-way solenoid valve which is located, hydraulically in parallel with the pump chamber and the inlet and outlet non-return valves connected to it, between the output of the main brake unit associated with the static brake circuit and the section of the main brake pipe of the rear axle brake circuit branching to the wheel brakes of the static brake circuit. The functional position of this 2/2-way solenoid valve associated with normal braking operation not subject to control is its through-flow position. The difference in volume of the pump chamber between its minimum and maximum values is at least 1/4 of the volume of that quantity of brake fluid which must be forced into the rear axle brake circuit in order to generate the maximum possible brake installation design pressure in the wheel brakes of this brake circuit. The electronic control unit for controlling brake pressure reduction phases of the antilocking control generates a combination of output signals by means of which the brake pressure control valve is switched into its shut-off position and the drive pressure space is initially connected to the tank of the auxiliary pressure source and is subsequently reconnected to its pressure output. The electronic control unit generates a signal combination for controlling the brake pressure retention phases, by means of which signal combination, the drive pressure space of the return pump is connected to the pressure output of the auxiliary pressure source and the brake pressure control valve is held in its shut-off position. For controlling brake pressure restoration phases, the electronic control unit generates a signal combination by means of which the drive pressure space of the return pump is connected to the high pressure output of the auxiliary pressure source and the brake pressure control valve is switched back into its through-flow position.
In the antilocking system of the invention, brake pressure reduction, brake pressure retention and brake pressure restoration phases controlled on the select-low principle, can be controlled in association with drive control of the return pump analogous to the known type of control for such a pump, by using a single brake pressure control valve designed as a 2/2-way solenoid valve. Relative to the otherwise necessary 3/3-way solenoid valve, this represents a substantial simplification relative to the known antilocking system in terms of both design and control. Since, from the statistical point of view, an overwhelming majority (more than 95%) of braking situations involve the brake installation being actuated with a force which corresponds to 1/4 or a smaller fraction of that actuation force which must be employed in order to generate the maximum possible brake installation design braking pressure, the increase in the pump chamber achievable by a single suction stroke of the piston of the return pump also suffices. In a correspondingly high proportion of brake situations requiring control, to achieve complete brake pressure reduction in the wheel brake or brakes subject to the control system, a repetition of the pump-back stroke of the return pump will only be necessary on rare occasions. In the large majority of braking actions subject to an antilocking control system therefore, the return pump also fulfills the function of the pressure reservoir provided in the known antilocking system, which can therefore be omitted in the antilocking system of the invention, which permits a further substantial reduction in the technical complication necessary for its achievement. The fact that given the statistical frequency quoted, a single pressure reduction and pump-back stroke of the hydraulically driven return pump is sufficient to achieve a brake pressure reduction appropriate to the situation, that is beneficial with respect to both the sensitivity of the control and the "control comfort" in the sense of a reduction of otherwise disturbing noise.
This applies--a forteriori--to the dimensioning of the return pump wherein the maximum volume change of the pump chamber is between 50% and 100% of the volume of that quantity of brake fluid which must be forced into the static brake circuit (II) in order to generate the maximum possible brake installation design pressure in both wheel brakes. This ensures that a single brake pressure reduction and pump-back stroke of the pump will be sufficient in every case to achieve a reduction in brake pressure appropriate to the situation.
An output signal, characteristic of the position of the piston of the return pump, from an electronic position indicator provided for this purpose, can be connected to the electronic control in an obvious manner to control the pump operation in such a way that brake pressure reduction displacements of the pump piston are limited to the amount necessary to combat the locking tendency, thus minimizing control cycle times.
If the drive control valve arrangement provides for connecting the drive pressure space of the return pump to the auxiliary pressure source and for shutting off the latter from the drive pressure space utilizing two 2/2-way solenoid valves, of which one is connected between the high pressure output of the auxiliary pressure source and the control connection of the pump drive pressure source and of which the second is connected between the tank of the auxiliary pressure source and this control connection, the basic position of the first 2/2-way solenoid valve being its open position and the basic position of the second 2/2-way solenoid valve being in its closed off position, the pump piston can, as it were, be "stopped" hydraulically in a simple manner at a specified position, e.g. one which can be monitored by the output signal of the position indicator; this can be an advantage for "comfortable" control of an antilocking control cycle.
Providing a non-return valve hydraulically in parallel with the brake pressure control valve and acted upon in the opening direction by a pressure in the main brake pipe of the static brake circuit which is higher than that in the associated output pressure space in the brake unit, and is otherwise shut off, has the effect that where the driver reduces the actuation force during a braking action subject to the antilocking control system, the brake pressure takes up the value selected by the driver.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.